[OKBODYMAN]

I am the proud owner of a FFR mk4 kit. It was given to me as a gift from my employer for 30 yrs of service. Unfortunately i have allready hit my first snag. I purchased a 2003 mustang cobra IRS assy, only to find that it will not bolt right in. I never read anything in the assy manual indacateing that it would or would not work. Has anyone else ever made this mistake? If so, how did you fix it? My thought is to modify the frame, but i'm not sure? Any help would be appreciated.